1. Field
The presently disclosed subject matter relates to an optical semiconductor device such as a light emitting diode (LED) and its manufacturing method.
2. Description of the Related Art
Generally, in a prior art method for manufacturing an optical semiconductor device, at least an n-type semiconductor layer, an active semiconductor layer and a p-type semiconductor layer are sequentially and epitaxially grown on a semiconductor growing sapphire substrate to obtain a semiconductor laminated body. Then, two or more different metal layers are sequentially deposited at a definite depositing rate on the p-type semiconductor layer to obtain a first metal laminated body. On the other hand, two or more different metal layers are sequentially deposited on a support substrate to obtain a second metal laminated body. Then, the first metal laminated body is bonded by a wafer pressure-welding bonding process to the second metal laminated body by inserting one or two adhesive layers therebetween. Finally, the growing sapphire substrate for absorbing a visible light component of light emitted from the active semiconductor layer is wholly removed by a laser lift-off process (see: JP2004-266240A and JP2004-281863A).
In the above-described prior art manufacturing method, however, when the metal layers of the first metal laminated body are sequentially deposited at a depositing rate higher than 5 Å/s, for example, in order to decrease the production time, each of the metal layers of the first metal laminated body has a low density portion or a coarse portion. As a result, the contact characteristics between the metal layers of the first metal laminated body would deteriorate, so that the metal layers would be peeled due to the difference in thermal expansion coefficient therebetween after the wafer pressure-welding bonding process.
Note that, if metals, whose differences in thermal expansion coefficient are small, are selected as the metal layers of the first metal laminated body, the peeling of the metal layers of the first metal laminated body would be avoided; however, in this case, the freedom of selection of such metals is decreased. That is, each of the metal layers of the first metal laminated body is not only intended to achieve the improvement of adhesion to the support substrate, but also, to achieve the improvement of the ohmic contact characteristic with the other layers and the contact characteristic with a reflective electrode layer, and suppress the diffusion of eutectic alloy in a eutectic alloy forming process. Therefore, since the freedom of selection of the metal layers of the first metal laminated body per se is small, this freedom of selection would be further decreased.
On the other hand, in the above-described prior art manufacturing method, when the metal layers of the first metal laminated body are sequentially deposited at a depositing rate lower than 1 Å/s, for example, each of the metal layers of the first metal laminated body has a high density portion or a tight portion. As a result, the contact characteristics between the metal layers of the first metal laminated body would be improved; however, in this case, a large residual thermal stress would be generated in each of the metal layers due to the difference in thermal expansion coefficient therebetween after the wafer pressure-welding bonding process. As a result, the first metal laminated body would be warped, so that the laser in the laser lift-off process would be not focused at the interface between the semiconductor laminated body and the growing sapphire substrate, which could not remove the growing sapphire substrate. Additionally, since the production time would be increased, the manufacturing cost would be increased.
When a first metal body including a single metal layer is provided instead of the above-mentioned first metal laminated body, similar problems would occur. In this case, if the single metal layer has a low density portion or a coarse portion, the contact characteristics between the single metal layer and its upper and lower layers would deteriorate, so that the single metal layer would be peeled due to the difference in thermal expansion coefficient therebetween after the wafer pressure-welding bonding process. On the other hand, if the single metal layer has a high density portion or a tight portion, the contact characteristics between the single metal layer and its upper and lower layers would be improved; however, a large residual stress would be generated in the single metal layer due to the difference in thermal expansion coefficient therebetween after the wafer pressure-welding bonding process. As a result, the single metal layer would be warped, so that the laser in the laser lift-off process would not be focused at the interface between the semiconductor laminated body and the growing sapphire substrate, which could not remove the growing sapphire substrate. Additionally, the manufacturing cost would be increased.